为什么这个块会抛出错误

"并非所有代码路径都返回值"

 public BallData GetBall(String Name)
    { 
        //Check each item in the list for the name.
        foreach (BallData Item in _BallList)
        {
            //If the name matches, return the item to the caller and exit the loop.
            if (Item.Name == Name)
            {
                return Item;
            }
            else
            {
                // Otherwise, throw an exception to indicate that the ball wasn't found.
                throw new KeyNotFoundException("The ball name doesn't exist.");
            }

        }
    }